As shown in FIG. 1, generally, speech signals communicated between a cellular phone (MS) and base station apparatus (BS) are sent on a bearer channel, and the bearer channel is of usage-based charging. Meanwhile, IP packets communicated between a personal computer and base station apparatus are sent on a packet channel, and the packet channel is of flat-rate charging.
Further, as shown in FIG. 2, the quality of communication is determined by allowable delay time and allowable jitter time, and higher quality is required in fixed-line phone and cellular phone than that of IP packet.
In recent years, the quality has been improved by technical progress such as absorption of jitter, reduction in delay time, measures against packet loss and the like, and it has been possible to communicate data while guaranteeing the same degree of quality in the packet channel as that of fixed-line phone and cellular phone.
Therefore, telephones have become widespread using the packet channel such as VoIP (Voice Over IP) and the like. It is expected that the number of users rapidly increases in the feature who use the inexpensive packet channel for speech call.
[See Patent Document 1] National Publication of International Patent Application No. 2002-533030